(a) Field of the Invention
The present invention relates to a photographing zoom lens system, and more specifically to a compact large-aperture zoom lens system suited for use with electronic photographing cameras, television cameras or the like.
(b) Description of the Prior Art
Zoom lens systems have hitherto been adopted as lens systems for almost all the electronic photographing cameras, television cameras or the like. So much considerations have not been taken for compactness and manufacturing cost of these zoom lens systems though they are more excellent than the lens systems for use with cameras using Leica size of film in that the former lens systems have aperture ratios higher than F/2.0 and varifocal ratios higher than 3. However, in the recent circumstance where popular photographing cameras adopt the electronic pick-up devices, compactness and reduction of manufacturing cost are required for optical systems for use with said popular photographing cameras. Therefore, demands are increasing for zoom lens systems comprising a front lens component with a small diameter, having a short total length and using a small number of lens elements.
Since the lens systems of this type have a characteristic as one of component parts consisting a photographing systems and the weight of such lens system is restricted for the purpose of performing automatic focusing and power zooming at low power consumption, it is desirable to adopt a rear focusing system in place of the focusing system by shifting the first lens group, reduce number of lens groups to be shifted for zooming and use a relay lens system prolonging the back focal distance.
There are many photographing zoom lens systems which have aperture ratios higher than F/2.0 and varifocal ratios higher than 3. However, there has conventionally been known no zoom lens system that can satisfy the three requirements of compactness (short total length and small diameter of the front lens component), low manufacturing cost (especially, a small number of lens elements) and a long back focal distance, and has performance high enough to cope with the future possible increase in number of picture elements on the image pick-up device. Especially, there has been obtained no high performance lens system that is designed for a back focal distance having a value exceeding 0.9 times of geometrical mean f.sub.S of the shortest focal length f.sub.W and the longest focal length f.sub.T of said lens system. Lens systems having a long back focal distance are required since it is necessary to interpose various optical members between the final lens surface of the system and photographing surface. Therefore, ratio between back focal distance f.sub.B and f.sub.S must be increased as focal length of the lens system becomes shorter. Japanese published examined patent applications No. 40852/80, No. 32387/73, etc. disclosed zoom lens systems comprising small numbers of lens elements and having short back focal distances, but took no considerations to prolong the back focal distance.
The conventional zoom lens system performed focusing by shifting the front lens groups having the large diameters and heavy weights. However, the focusing system of this type had a defect to allow remarkable variations of aberrations, especially spherical aberration and chromatic aberration, especially when the lens systems are focused on objects located at short distances on the long focal length sides of the lens systems. Further, the focusing system of this type has another defect that diameter of the front lens component must be increased since the lower rays are eclipsed by the front lens component when the lens system is focused on an object located at a short distance. In order to eliminate these defects, it is preferable to adopt the rear focusing system. Automatic focusing is made possible at low power consumption by adopting the rear focusing system. As a zoom lens system using the rear focusing system, there has been known the one disclosed by Japanese published unexamined patent application No. 78513/82.
In the lens system disclosed as Embodiment 1, however, aberrations, especially astigmatism, are remarkably varied when said lens system is focused on an object located at a short distance. The lens system disclosed as Embodiment 2 has a small aperture ratio of F/5.6. Therefore, this conventional example does not satisfy the above-mentioned requirements of an aperture ratio larger than F/2.0 and a back focal distance longer than 0.9f.sub.S. In addition, performance of the said lens system is not so high since it allows remarkable variations of aberrations, especially spherical aberration and coma, when it is focused on an object located at a short distance.